You will often find cheaper fares on many airlines from either or both of these, not just NW, in what's sometimes called the "Southwest effect." If you live north or south of Boston itself, it may even be easier to get to one or the other than to Logan. However, while that's the general rule, it has exceptions, so check each time.

The big downside is that, if your travel plans change while you're away, you have fewer options for getting back. (If you don't drive to the airport, that's less of a concern.)